Method of and system for the controlling of an apparatus for the electric discharge machining of a workpiece

ABSTRACT

Discrete electric-discharge pulses are applied across a dielectric-swept gap between an electrode and a workpiece by triggering an ON-OFF switch between its conductive and nonconductive states in accordance with parameters of the gap so that per pulse adaptive control of the individual machining discharges is effected. An integrator responsive to the gap current, may be used to operate the switch means which may be turned on upon the detection of a pilot discharge. The number of unsatisfactory discharges is detected and, if this number exceeds a predetermined level, is used to control a gap parameter. The integrator signal may be digitalized for accumulation in a counter controlling pulse duration and other parameters.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a division of application Ser. No. 408,382,filed Oct. 23, 1973, now U.S. Pat. No. 3864541, Feb. 4, 1975; which is acontinuation-in-part of application Ser. No. 272,463, July 17, 1972,U.S. Pat. No. 3,781,507, Dec. 25, 1973; which is a division ofapplication Ser. No. 19,364, Mar. 13, 1970, U.S. Pat. No. 3,686,461,Aug. 22, 1972; which also dealt with subject matter in part disclosed inapplication Ser. No. 838,575, filed 2 July 1969 (U.S. Pat. NO.3,604,885); which is a continuation-in-part of then pending applicationSer. No. 682,824, filed 14 Nov. 1967 (U.S. Pat. No. 3,539,755) and as acontinuation-in-part of a still earlier application Ser. NO. 493,473 of6 Oct. 1965 (U.S. Pat. No. 3,360,683). The present application alsorelates to my U.S. Pat. No. 3,875,374, which makes references to U.S.Pats. Nos. 3,539,755 and 3,536,881 as well as to U.S. Pats. Nos.3,604,885 and 3,686,461 which issued on applications copending with theparent application hereof.

FIELD OF THE INVENTION

The present invention relates to electric-discharge machining (EDM)systems and, more particularly, to a method of and an apparatus orsystem for controlling the machining pulses of an EDM apparatus.

BACKGROUND OF THE INVENTION

In electric-discharge machining (EDM), an electrode is spacedlyjuxtaposed with a workpiece across a gap through which a dielectricliquid is forced while a power supply capable of providing electricalpulses sufficient to effect dielectric breakdown across the gap isconnected in circuit with the electrode and the workpiece. With eachdischarge, a portion of the workpiece is eroded and the detritus iscarried away by the dielectric liquid and a nonconductive orlow-conductivity state is re-established in the gap.

In early systems of this type, the machining-pulse source was acapacitor connected across the gap and charged by a direct-currentsource connectable in parallel therewith, the gap breaking down andsustaining the machining discharge when the potential across theelectrodes reached the breakdown potential of this gap. These systemshad the disadvantages that the discharge-pulse energy varied fromdischarge to discharge, that short-circuiting prevented the buildup ofthe capacitor charge while conditions close to short-circuitingprevented significant energy storage in the capacitor, and thatsignificant opening of the gap prevented any discharge whatsoever.

To overcome these disadvantages, switch-pulse systems have been employedwherein an electric-discharge machining current source is connected incircuit with the gap by ON-OFF switch means of an electronic, mechanicalor electromechanical type and the switch means is altered in statebetween its conductive and nonconductive conditions to apply asubstantially rectangular waveform discharge pulse across the gap. Inprior systems of this gendre, the switch means was controlled by amultivibrator or other ON-OFF signal generator and it was quickly foundthat the parameters of the electrical pulse had to be established mostcarefully for efficient machining, especially where electrode wear is tobe limited ("no wear") and energy loss is to be a minimum.

Various disturbances arise in electric-discharge machining and, sincethey have been discussed in some detail in my earlier applicationsmentioned above, it is necessary only to refer to several of them toplace the present developments in EDM pulse control in the properperspective. It is desired to have the pulses or discharges be of equalenergy from pulse to pulse (isoenergy pulses). However, detritus doesnot always clear uniformly from the gap and residual ionization may makehigh energy pulses premature from time to time. The tool electrode mayapproach the workpiece too closely so that a particular discharge energymay be excessive or the gap may be short-circuited in which case thedischarge will merely exacerbate the problem. Finally, arcing(continuous discharge) may occur if insufficient gap recovery time ispermitted or some other gap parameter is altered. Consequently, withmultivibrator or like control of ON-OFF time for the discharge pulse, itwas also proposed to vary the pulse duration, the interval between thesuccessive pulses and the pulse height on a per-pulse basis. It was alsofound, as discussed in the aforementioned applications, thatself-adpative control on a per-pulse basis was desirable, in which casemeans was provided to detect a gap condition indicative over the statusof the gap during the next discharge period for controlling thesubsequently developed discharge is to amplitude, duration or interval.

OBJECTS OF THE INVENTION

It is an important object of the present invention to extend theprinciples set forth in the copending applications mentioned above andfurther improve the discharge machining systems set forth therein.

Another object of the invention is to provide an improved method ofcontrolling the discharge pulses of an EDM system on a per-pulseadaptive basis.

It is also an object of this invention to provide an improved system formachining a workpiece under EDM principles with an improved uniformity,accuracy and efficiency and with reduced downtime or tendency towardinterruption of machining.

Still another object of the invention is to provide a more effectiveparameter-control system for an EDM arrangement.

SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter areattained with an EDM system in which, according to an important featureof the invention, the machining current during the discharge isintegrated to control an ON-OFF switch means. The integrator mayintegrate the entire machining current and provide a signal representingthe current integral or may provide an integrated current signal otherthan the machining current signal but traversing the gap as will beapparent hereinafter. An important advantage of this concept is that atruly square wave or rectangular waveform pulse may be applied by theswitch means across the gap. The switch means is thus controlled (i.e.shifted between its ON and OFF states) in response to the charge anddischarge characteristics of a capacitor connected across the gap andserving as an integrating system without contributing materially to thecurrent flow through the gap during the machining discharge. Thus thecapacitor charge and discharge determines the duration of the machiningdischarge and the discharge repetition rate while the machining energyis supplied directly from a machining power supply through the ON-OFFswitch.

Thus a method of controlling the parameters of a train of pulses appliedacross a dielectric-swept gap between a machining electrode and aworkpiece may involve charging a capacitor connected in parallel withthe gap at a rate determined by the recovery of the gap from adischarge, and triggering substantially instantaneously the switch meansin accordance with the potential across the capacitor while maintainingthe current contribution of the capacitor to the machining current pulseat a small fraction of the latter.

According to this aspect of the invention, a system for the electricdischarge machining of a workpiece may comprise the electric-currentsource for producing a rectangular-wave machining current pulsesubstantially instantaneously upon the triggering of a switch means incircuit with the source and the gap and a capacitor connected inparallel with the gap and chargeable at a rate determined by therecovery characteristics thereof while contributing at mostinsignificantly to the discharge current pulses.

A discriminator, preferably a Schmitt trigger circuit, is connected tothe capacitor and is responsive to the charge level thereof forsubstantially instantaneously triggering the switch means into one ofits conductive or nonconductive states. The Schmitt trigger may have twothresholds for triggering the switch means into one of the states uponthe capacitor potential attaining one of the thresholds and into the onestate upon the potential falling to the other threshold. A firstvariable resistor and a diode in one sense is connected as a chargingnetwork with the capacitor across the gap while a second variableresistor and a diode poled in the opposite sense is connected in serieswith the capacitor across the gap as a discharge network. A Zener diodemay be applied across the capacitor for limiting the voltage buildupthereacross. The capacitor may be charged with current derived from themachining-current source or from a second source connected across thegap to effect a breakdown thereof. Furthermore, a switch means ispreferably connected across the capacitor and is switched into aconductive state when the machining-pulse switch is open circuited todrain the capacitor and prevent residual charge buildup therein.

According to another aspect of the invention, the control systemincludes means for converting the control signal, e.g. the charge uponthe capacitor, into signals for initiating a train of discrete pulsesand terminating this train of pulses, the pulse train, in toto, formingthe signal applied to the switch means and thereby producing acorresponding discharge current pulse.

According to the invention, a signal representing the gap current andpreferably the gap current itself is integrated after initiation of amachining discharge to switch the power supply upon the integral signalattaining a predetermined value. Preferably a low-current high-voltagepower supply is applied across the gap to initiate a pilot discharge bybreakdown of the dielectric and the machining power supply orlow-voltage high-current source is switched on to produce the dischargepulse of generally rectangular waveform upon detection of the pilotdischarge. The gap current is thereupon integrated to produce thecontrol signal which turns off the machining current power supply.

The signal representing the gap current may be converted into a train ofdigital pulses which are integrated, e.g. by accumulation in a capacitoror by accumulation in a digital counter of the solid state type toproduce the control signal.

According to another aspect of the invention, the duration of eachmachining discharge is timed and a number of unsatisfactory machiningdischarges is counted over a predetermined sequence of machiningdischarges, either by ascertaining the number of satisfactory dischargedor by ascertaining the number of unsatisfactory discharges, and aparameter of the gap is modified upon the count of unsatisfactorymachining discharges exceeding a predetermined number.

Since reference is made herein to modification of a gap parameter it isto be understood that such modification may be made in accordance withthe techniques described in application Ser. No. 338,849 (U.S. Pat. No.3,875,374) dealing with EDM parameter control systems. In other wordsthe system of the present invention may be used with an EDM parametercontrol system as described in the latest application.

The "unsatisfactory discharges" are those having a duration outside apredetermined duration range and, most frequently, those having aduration less than a predetermined minimum duration previouslydetermined to be the minimum effective discharge duration for efficientmachining.

The timing of the duration of each machining discharge may be effectedby differentiating the machining pulse or the switch-on pulse to triggera timer into operation, the output of the timer being monitored by acomparator to which a reference time input is supplied, the count beingoperated by the comparator to respond either to a discharge having ashorter period or a discharge having a longer period. In either case, alogic circuit may be provided to control the pulses transmitted to thecounter, an AND gate being used where the counted pulses are to bepassed and an INHIBIT gate being used where the respective pulses arenot to be transmitted to a counter.

The integral control of the present invention has an advantage over gapindependent timing of the on-off states of a power switch in that themachining efficiency of the latter may be low because of failure tosupply sufficient energy during some pulses and excessive energy forothers depending upon the gap conditions. Attempts to apply a series ofvoltage pulses of fixed width resulting in a series of discharge currentpulses of longer and shorter width independently of the gaprequirements.

With integral control the repetitive pulse discharges and a variablepulse width which inversely depends upon the varying gap currentmagnitudes and hence are substantially of equal energy per pulse (energyE being represented by the relationship ##EQU1## The energy value perpulse is generally maintained in spite of varying gap conditions and thepulse is terminated when the integral reaches a preset level in terms ofthe control signals and hence in terms of the energy E per pulse. Ofcourse, instead of the change in the current signal for detecting thedischarge initiation or gap breakdown a change in the gap voltage may beused to indicate triggering of the discharge.

The integrator control thus provides a succession of discharge pulseswith uniform or substantially equal energy.

Furthermore, the integral control system of the present invention, usinga pilot pulse triggering of the discharge, creates a current pulse whichis exactly in coincidence with the voltage pulse applied by themachining current source. Delays resulting from lack of phasecoincidence are thereby eliminated.

The present invention also provides, as has been noted, for the countingdirectly or indirectly of unfavorable discharge pulses for comparisonwith a reference value, e.g. a signal representing the occurence offavorable discharge pulses counted separately, to provide a signal whichindicates machinig efficiency. It will be immediately apparent that aratio of 0 unfavorable machining pulses to 100% favorable machiningpulses indicates maximum machining efficiency while a large number ofunfavorable machining pulses and a small number of favorable pulses willindicate poor efficiency.

The exclusion of unfavorable discharge pulses to the greatest possibleextent has been found to be highly desirable for so-called "no-wear"operation in which wear of the tool electrode is to be minimized. Withan isoenergy pulse system as described above, it has been found to beadvantageous to detect the frequency of unfavorable discharges andthereby ensure that the greatest possible proportion of discharge pulseshave a greater magnitude identical to or closely approaching a desiredvalue preselected in accordance with the machine operating mode. Hencein addition to providing the isoenergy system, I make use of logiccircuitry to ascertain the frequency of unfavorable pulses and controlthe parameters of the gap.

According to still another aspect of the invention, the dischargemachining apparatus comprises means for deriving an analogous signalrepresentative of an electric current passing through the machining gap,a converter for transforming the analog signal into narrow digitalpulses of a pulse train, a counter for counting the digital pulses and agate circuit operated by an output signal of the counter for terminatingeach of the gating signals applied to the switch means for triggeringthe latter on and off. It is important in this latter mode of operationthat the counters respond to respective clock pulses generated fromseparate sources.

Each discharge pulse is thus converted into digital signals controlledby a signal derived from monitoring the gap characteristics andaccumulated or integrated. Upon the integrated or accumulated valuereaching a predetermined value, switch control results. Accordingly, thepulses have a uniform energy with the width being increased or decreasedin dependence upon the variation in gap current and in accordance withvarying gap characteristics. A selector switch may be provided to setthe counters, which may be conventional digital pulses countersoperating off an analogous-digital converter (A/D converter) to set theapparatus for the particular machining operation.

An off-time setting circuit may be provided to reset the gate fortriggering the on-off switch according to still another feature of theinvention. In accordance with this concept, a timer circuit establishesa checking time following resetting of the gate circuit while a delaycircuit is provided to delay the resetting of the gate circuit when thecount on the counter reaches a set value before the lapse of thechecking time. The latter system is thus capable of ascertainingsatisfactory and unsatisfactory pulses for control of gap parameters orof the power supply.

DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIG. 1 is a circuit diagram, partially in block form, of an EDM powersupply using a low-capacitance capacitor for operating an on-offmachining pulse switch according to the present invention;

FIG. 1A represents a modification of the system of FIG. 1 but showscircuit components which may be utilized in conjunction therewith,including a separate gap-breakdown current source;

FIG. 1B is a circuit diagram illustrating another modification of thecircuit of FIG. 1;

FIG. 2 is a diagram of a Schmitt-trigger discriminator for use in thepower supply system of FIG. 1 and elsewhere in accordance with thepresent invention;

FIG. 3 is a graph showing waveforms of the system of FIG. 1;

FIG. 4 is a circuit diagram, partially in block form, representinganother modification of the capacitor control system of the invention;

FIG. 5 shows a circuit operating generally similarly to the system ofFIG. 1 but using a separate switch for draining the capacitor;

FIG. 6 is a circuit diagram of an EDM power supply embodying the featureof FIG. 5 and in addition having a separate source for charging thecapacitor;

FIG. 7 is a circuit generally similar to FIG. 6 but containing anadditional feature of the invention;

FIG. 8 is a capacitor control circuit according to the invention inwhich the ultimate control signal is a train of discrete or digitalizedpulses;

FIG. 9 is a circuit diagram of an EDM control system in which thecontrol signal is derived by integrating the current through thedischarge gap from the breakdown-voltage source;

FIG. 10 is a circuit diagram similar to FIG. 9 but showing the system inwhich the integrated current is derived from the machining-currentsource;

FIG. 11 shows a digital system according to the invention forcontrolling on and off time;

FIG. 12 is a circuit diagram for assisting in the explanation of thesystem of FIG. 11;

FIg. 13 is a graph showing the pulses of the system;

FIG. 14 is a block diagram illustrating another aspect of thisinvention,

FIG. 15 is a block diagram of a circuit according to the invention inwhich integration occurs as a summation;

FIG. 16 is a block diagram of another gap-responsive control systemusing, as in FIG. 15, the variable frequency output of an analog/digitalconverter;

FIg. 17 is a circuit diagram of the converter of FIG. 15 or FIG. 16; and

FIG. 18 is a circuit diagram of still another control system using ananalog/digital converter.

SPECIFIC DESCRIPTION

In FIG. 1A I have shown a circuit for the servocontrol of the gap of anelectrical discharge machine which operates on the adaptive controlprinciple disclosed in my application Ser. No. 272,463 now U.S. Pat. No.3,781,507, mentioned earlier and which has circuit elements in commonwith the pulse-control systems of the present case so that this circuitwill be described in some detail herein.

The circuit of FIG. 1A uses an adaptive EDM pulse generator 25 asoriginally illustrated in my U.S. patent application Ser. No. 838,575now U.S. Pat. No. 3,604,885, which was copending with and is referred toin said application Ser. No. 272,463, now U.S. Pat. No. 3,781,507. Means26 is provided for short circuiting the gap through a shunt resistor 26awhen the gap voltage falls below a predetermined value, say, 10 volts.

The servocircuit of FIG. 1A has a basic form and operation essentiallyidentical to that shown in FIG. 1 of application Ser. No. 272,463, nowU.S. Pat. No. 3,781,507, and is provided with a threshold Schmitttrigger 412 with a variable impedance means, here constituted by a NPNtransistor tr1 connected across the resistor 412r to control, coupledwith the latter, a threshold value of servo-operation in response to asignal derived from a comparator 27 as will be apparent.

A gap-current monitor 28 is provided which includes a differentialtransformer 28a whose primary winding is connected across a variable tapand a fixed terminal of a gap resistor as shown (the gap resistor isconnected in parallel with the gap, i.e. with its outer terminalsconnected to the workpiece and to the electrode). The secondary windingof this transformer is connected via a full-wave rectifier 28b across anintegrating capacitor 28c whose positive terminal is, in turn, connectedto the base of a NPN transistor 27a in the comparator 27. Thus it willbe apparent that the terminal voltage represents a gap signal indicativeof characteristic gap conditions or, in electrical discharge machining,the degree of occurrence of liquid-phase discharges, gaseous discharges,liquid-gas mixed phases discharges, arcing (continuous discharge), shortcircuiting (no gap), etc.

A higher capacitor terminal voltage indicates a higher proportion ofnormal discharges occurring while a lower integrating or terminalvoltage indicates a condition tending to shift into continuous arcing orshort circuiting.

The comparator 27 is here constituted by a differential amplifier whichincludes a fixed voltage source 27b, NPN transistors 27c and 27d, and aninput signal transistor 27a along with resistor connections as indicatedin FIG. 1A.

Resistor 27e is variable to establish at the collector of transistor 27c(a predetermined reference voltage depending upon the particular type ofmachining operation to be conducted and, here, also corresponding to anoptimum gap condition for comparison with the collector voltage oftransistor 27a which is variable in accordance with the actual gapcharacteristics. The comparison or difference signal thus derived as fedto the control electrodes of transistor Tr1 to vary thecollector-emitter conductivity thereof and hence the threshold value ofthe Schmitt trigger 412 in accordance with and as a function of themagnitude of the comparison signal.

In operation, it will be seen that when machining is carried on in anoptimum mode, the integrating voltage of the capacitor 28c is high,applying a greater control signal to the base of transistor 27a whosecollector/emitter resistance and hence its collector voltage, is held ata value substantially balancing the collector voltage of transistor 27c.As a consequence, transistor Tr1 is held nonconductive or substantiallynonconductive and the threshold Schmitt circuit 412 derives the inputgap analoge signal across resistor 412r, the signal being applied to thefirst transistor 412a of the Schmitt trigger. In this state, it will beseen that the system tends to hold the machining gap at a minimum presetspacing (with operation of the servomotor as described in applicationSer. No. 272,463, now Pat. No. 3,781,507) so as to facilitate metalremoval or to allow machining to progress at maximum speed.

When, however, the gap condition worsens or gaseous discharge or a likecondition appears, the collector voltage of transistor 27a rises abovethe collector voltage of reference transistor 27c because of a decreasein the terminal voltage of the gap responsive integrating capacitor 28c.The resulting difference signal from comparator 27 renders thetransistor Tr1 conductive at a collector/emitter resistance determinedby the magnitude of the difference signal. The transition level ofSchmitt trigger 412 is lowered and the system is thus controlledadaptively to tend to maintain a relatively wider gap spacing asdetermined by the lower servo threshold level so that the optimumcondition can be restored and development into continuous arcing or alike damaging condition is avoided.

The thresholder controller for the pulse width of the machining currentincludes a transistor Tr2 constituting a variable impedance meansprovided across the threshold resistor r_(o) or an input resistor r_(o)' of a Schmitt trigger pulse shaper in the power supply 25. Thedifferential output of comparator 27 is also applied to the controlelectrodes of this latter transistor so that, for optimum machiningconditions, the machining pulse width is held at a maximum preset valueand as the gap condition worsens the pulse width is decreasedaccordingly to avoid further development into an arcing or likecondition.

The power supply shown in FIG. 1A is, as noted, a modification of thesystem described and claimed in application Ser. No. 838,575 U.S. Pat.No. 3,604,885 and likewise disclosed in application Ser. No. 272,463,now U.S. Pat. No. 3,781,507. This system includes a gap-conditiondetector connected across the gap and represented by the lead 250between a capacitor 25b₁, and a resistor 25b₂. The capacitor is bridgedby Zener diode 25b₃.

The sensed signal, indicating the gap condition, charges the capacitator25b₁ or bucks the charge delivered from some other source so that anintegrated signal is applied to the base of transistor 25c₁ of theSchmitt trigger circuit 25c. The signal applied to the control electrodeor base of this transistor is delivered through the resistor r_(o),which may constitute a variable impedance as previously noted or via atransistor Tr2' whose principal electrodes may be connected in serieswith the base of transistor 25c₁ and the integrating network 25b.

The emitter/collector network of the Schmitt trigger transistor 25c₁includes the variable impedance system r_(o) and Tr2 controlled by thecomparator 27. Between the collector of transistor 25c₁ and the base ofthe conjugate transistor 25c₂ there is provided the usual time constantor delay network 25c₃. The output of the Schmitt trigger is here derivedacross the resistor 25c₄ to which the collector/base terminals of anamplifying and phase-reversal transistor 25d is connected.

The integrated signal is applied to the base of the first transistor ofthe Schmitt trigger 25c, the thresholds of which are set by thevariable-impedance network r_(o), Tr2 as previously described. Theoutput transistor applies its signal to the amplifying and phasereversal transistor 25d, the latter in turn energizing a switch in twostages as represented at 25e and 25f. The Schmitt trigger 25c may be ofthe type described at page 389 ff of PULSE, DIGITAL AND SWITCHINGWAVEFORMS, McGraw-Hill Book Co. 1965, and acts as a discriminatorconverting the level of the input analog signal of the integratingcircuit 25b to a digital output represented by the two stages of theSchmitt trigger. The two-stage switch comprises a transistor 25e whoseemitter is connected in parallel with the bases of the transistors 25f₁,25f₂, 25f₃, etc., the principal electrodes of these latter transistorsbeing connected in parallel between a power source 25g, represented as abattery, and the electrode gap in series with the tool electrode E andthe workpiece electrode W.

Zener diodes 25f₄, 25f₅, and 25f₆ are connected across theemitter/collector terminals of the power transistors 25f₁, 25f₂ and 25f₃to prevent overloading thereof. The machining current is, of course, afunction of the number of power transistors of this latter network whichare connected in parallel between the source and the electrodes. Anelectronic switch 25h is also connected in series with the directcurrent source 25g and is represented as a silicon controlled rectifier25h, whose gate is triggered to turn on the power circuit.

As noted earlier, a shunt 26 is provided to short circuit the electrodesE, W as controlled by a gap voltage detector 26a, consisting of a pairof resistors 26a₁, 26a₂, forming a voltage divider, the tap 26a₃ ofwhich is applied to the base of a transistor 26b₁ forming a part of aSchmitt trigger circuit 26b as describbed in PULSE, DIGITAL ANDSWITCHING WAVEFORMS cited earlier. Between the base an collector ofsecond or conjugated tranistor 25b₂ there is provided a time delay ortime-constant network 26b₃. The output of the Schmitt trigger, whichdigitally controls the shunting operation, is developed across theresistor 26b₄ and is applied to the base/collector terminals of anamplifier and phase reversal transistor 26c. The output of the latter isapplied via a potentiometer 26d to the base of the transistor 26e whoseprincipal electrodes are connected in shunt across the workpieceelectrode W and the tool electrode E. When the gap voltage drops below10 volts, the Schmitt trigger 26b reverses in state and, via atransistor 26c, renders a transistor 26e conductive to short circuit thegap.

In FIG. 1 I have shown, somewhat more diagrammatically, a basic circuitaccording to the principles of the present invention wherein a capacitoris provided across the gap to control an on-off switch means inaccordance with the present invention without a material contribution tothe gap current from the capacitor. The capacitor here acts, as in theembodiment of FIG. 1A in which it is part of an integrator circuit, asthe sensing element.

In the system of FIG. 1, the tool electrode 1 and the workpiece 2defines a gap G which, as in the case with the gap of FIG. 1A, may beflooded with dielectric using a circulation system of the typeillustrated in FIG. 1B to be described subsequently. Across this gap Gthere is connected a charge/discharge capacitor 3 shown to be of thevariable capacitance type and effectively connected in parallel with themachining gap. The capacitor 3 may be charged through a resistor R₁ anda rectifier diode RC₁ forming a charging network 4 and disposed betweenthe positive terminal of a machining current source 6 represented as thebattery and a negative terminal thereof. Since the electrode 1 and theworkpiece 2 are respectively connectably to the positive and negativeterminals of the direct current machining source 6, the network 3, 4lies directly in parallel with the gap. The capacitor 3 may dischargethrough a network 5 consisting of a variable resistor R₂ and a rectifierdiode RC₂ poled oppositely to the diode RC₁ described earlier. Thedischarge circuit is closed through the gap G.

The machining-current source 6 lies in series with a switch 7 which isshown as a mechanical switch merely for purposes of illustration. Inpractice, the switch 7 may be a transistor or transistor bank asdescribed for the switching circuit 25 of FIG. 1A and is alsoillustrated diagrammatically in FIG. 1B. It also may be constituted by athyristor such as that shown at 25h₁ in FIG. 1A, or electron tubes orother high-speed on-off switch means.

The direct current source 6 may be constituted as a single source ofmachining current, in which case the switch 7 may have a high-resistive7a thereacross to enable the capacitor 3 to be effective, or may bedivided into two sources in parallel with one another as illustrated inFIG. 1B. In this case, the machining current source is a low-voltagesource designed to provide a high current for machining at, for example,a voltage level of less than 100 volts and sufficient to sustain thedischarge over the duration of the machining poles but insufficient toinitate this discharge by breakdown of the gap. The high-voltagelow-current source is designed to provide a voltage sufficient toinitiate the discharge, for example several hundred volts, but does notmake a material contribution to the machining current pulse. The highvoltage or discharge-initiating source is connected in parallel with thevoltage source 6 and across the gap. The switch 7 is located in serieswith the low voltage machining-power source.

The circuit represented in FIG. 1 comprises a discriminator circuit 8(FIG. 2) adapted to respond to the capacitor 3 and discriminate betweencharge and discharge voltages to control the on and off operation of theswitch 7.

From FIG. 2 it will be apparent that the discriminator 8 is aSchmitt-trigger circuit of the type described generally in connectionwith FIG. 1A and as also discussed in PULSE, DIGITAL AND SWITCHINGWAVEFORMS cited earlier.

The circuit of FIG. 2, in which the capacitor 3 has been showndiagrammatically, comprises an input transistor T₁ of the NPN type whosebase is connected to one terminal capacitor 3 and whose emitter isconnected via the bias resistor R₃ to the negative bus of the system.The positive bus is connected by a threshold-setting resistor R₄ to thecollector of transistor T₁.

A signal-shaping network consisting of a capacitor C₁ in parallel to aresistor R₅ is connected between the collector of transistor T₁ and thebase of a transistor T₂ of the NPN type whose emitter is tied to theemitter of transistor T₁. A second threshold-setting variable resistorR₆ is provided between the positive bus and the collector of transistorT₂ which is also connected via a resistor R₇ to the base of an amplifiertransistor T₃. This NPN transistor has its collector connected by a biasresistor R₈ to the positive bus and by a resistor R₉ to the base of anoutput transistor T₄ while its emitter, like the emitter of transistorT₄, is connected to the negative bus. The bias resistor R₁₀ is connectedbetween the positive bus and the collector of NPN transistor T₄ whoseoutput is applied via a diode D₁ to the base of a transistor 7aconstituting an electronic switch adapted to be provided at the switch 7of FIG. 1. The Schmitt trigger circuit 8 of FIG. 2 thus has a pair ofconjugate transistors T₁ and T₂ and an amplifier circuit havingtransistors T₃ and T₄ which amplify and invert the outpt of the Schmittcircuit.

A Zener diode 9 is connected across the terminals of the capacitor 3(FIG. 1) to protect the latter against excessive voltage levels. After adischarge has terminated at the machining gap G, the capacitor 3 beginsto charge through the network 4 at a rate determined by the resistanceof resistor R₁ and the capacitance C of the capacitor 3 along the curverepresented at R₁ C in FIG. 3 in which the capacitor voltage V_(c) isplotted along the ordinate against t (time) plotted along the abscissa.Since the charging is effected from the source 6 through the ohmicresistance of switch 7 in the off state, no machining pulse passesduring this period. When the charge on the capacitor reaches apredetermined level, the state of the Schmitt circuit will invert andthe blocking transistor T₁ will be rendered conductive while transistorT₂ is switched to a nonconductive state from its previous conductingcondition. Transistor T₃ is thereby turned on and transistor T₄ isturned off to develop a collector signal which turns the switch 7, 7aon, allowing the source voltage to be applied directly to the machininggap.

Since the capacitor is always in circuit (parallel) with the machininggap, its charging rate is, of course, dependent upon the gapdeionization characteristics or gap impedance. When switch 7 is turnedon, the discharge proceeds with a substantially rectangular wavefront(see the voltage V and current I graphs of FIG. 3) and machining takesplace.

From FIG. 3 it will be evident that the time interval t.sub. i and t_(i)' between machining discharges varies in response to the dielectricrecovery characteristics of the gap which controls the instant ofdischarge initiation along the capacitor-charge accumulation asdescribed. When the discharge begins, the capacitor 3 begins todischarge through network 5 with its terminal voltage dropping along thecurve R₂ C reached in FIG. 3. This discharge characteristic is likewisevariable in accordance with the gap condition through which thecapacitor discharges.

During the discharge of capacitor 3, discriminator circuit 8 maintainsits inverted state so that switch 7 is held on to maintain the dischargecurrent through the gap. The discharge current across the gap fromsource 6 includes a contribution from the capacitor 3 but the latter isnegligibly small since the capacitor may have a small capacitance of upto several microfarads and is in series with the resistor R₂. As aconsequence the gap current is substantially constant notwithstandingthe discharge of capacitor 3.

When the capacitor voltage drops to a predetermined level as establishedby the threshold of discriminator 8, the latter switches to its originalstate to turn switch 7 off, thereby terminating a discharge cycle of themachining system. The resistors R₄ and R₆ set the upper and lowerphase-inverting levels of the discriminator and upon reversal of state,the discharge across the gap is terminated. The machinging current pulsewidth is thus dependent upon the capacitor discharge time and hencedecreasing the resistance of discharge resistor R₂ will narrow the pulsewidth whereas increasing it will lengthen the pulse width. The operatingconditions for particular machining operations with rectangular wavedischarges may thus be set as required. Of course, since the pulse widthis a function of the gap condition, the setting of resistance R₂ willhave little effect except during stabilized machining when the pulseduration is substantially constant. As soon as an unstable gap conditiondevelops, the adaptive characteristics of the present system operate toplace the pulse width under the control of capacitor 3. After thetermination of the machining discharge, capacitor 3 charges again inaccordance with the recovery characteristics of the gap, i.e. morerapidly or more slowly depending upon whether the gap recovery is fastor slow. The cycle is then repeated.

FIG. 3 also shows the phase relationship between the waveform C acrossthe capacitor 3, the voltage pulse V applied across the machining gapand the discharge current pulse 1 therethrough.

The capacitor 3 thus has a terminal voltage which varies in accordancewith changes in the machining-gap conditions and is used, through thediscriminator circuit, to turn the switch 7 on and off therebyimpressing the voltage across the machining gap for a durationdetermined by the on time of the switch 7. This system minimizes theinterval between successive pulses to that which is required forrestoration of a gap condition enabling the next machining pulse to beeffective. The average current is thus maintained at a high levelautomatically without permitting a damaging arc discharge.

It is important to observe that in the present system the discharge isnot derived from the capacitor but is delivered directly by the voltagesource 6 of predetermined output characteristics, simply by switchingthe latter on and off. The discharge current pulses thus have therectangular waveform described in conjunction with FIG. 3. Adjustment ofthe resistors 4 and 5 varies the slope of curves R₁ C and R₂ C of FIG. 3and thus allows the charging rates to be set at the desired levels forlow wear machining and other machining modes.

In FIG. 1B, I have shown a system which is generally similar to FIG. 1but employs additional circuit elements to be described hereinafter. Inall of the embodiments hereinbefore and hereinafter described, the toolelectrode 101 may be tubular and can be supplied with dielectric from apump 101a by a pipe 101b, the dielectric overflow being collected in atrough 101b, the dielectric overflow being collected in a trough 101cconnected by a pipe 101d to a reservior 101e provided with a filter orthe like. The pump 101a draws the dielectric from this reservoir. Inaddition, the servosystem 101f is connected with the electrode 101 (orthe workpiece 102) and can respond to the gap characteristics in themanner described in my application Ser. No. 272,463, now Pat. No.3,781,507.

The system of FIG. 1B comprises the machining current source 106connected in series with a rectifier diode 106a and an electronic switch107 having ganged transistors 107a of the type shown in FIG. 1A andcontrolling the level of the machining current in accordance with thenumber of transistors which are effective. A Schmitt trigger circuit108, identical to that of FIG. 2, is connected across the sensingcapacitor 103 and the Zener diode 109 in the manner previouslydescribed. In addition, a transistor 103a has its emitter collectorterminals connected to the output of the discriminator so as to renderthe transistor 103a conductive when the switch 107 becomes nonconductiveand drains the capacitor of any residual charge. The charging network104 and the discharging network 105 of this embodiment correspond tonetworks 4 and 5 previously described.

The output of the threshold discriminator 108 is also applied through asensor resistor 103b having a differentiator network 103c connectedthereacross to indicate the leading edge of the signal initiating themachining pulse. A pulse timer 103e provided with a suitable clock (notshown) is connected across an integrating network 103d receiving thecontrol signal so that a gating circuit 103f passes signals reachingeach discharge pulse to store, at counter 103g, a signal representingthe number of satisfactory pulses or the number of unsatisfactory pulsesas will be described in greater detail hereinafter. The counter 103gproduces a control signal which may be applied to the dielectricrecirculation system as represented at 103h or to the servocontrolsystem as represented at 103i to adjust the gap parameter when thenumber of unsatisfactory pulses in a given sequence exceeds apredetermined minimum. The control signal may also be applied to aswitch 103j, 103k cutting off machining operation should the ratio ofunsatisfactory pulses to satisfactory pulses be intolerably high. Thedischarge is initiated by a pilot voltage source 110 through a rectifierdiode 110a. The system of FIG. 1B, of course, operates similarly to thatof FIG. 1.

FIG. 4 shows an embodiment of the invention in which the capacitor 203is connected in series with a discharge network 205 containing theresistor R₂ and the diode RC₂ across the gap formed by the electrode 201and the workpiece 202. A separate direct current source 210 is providedin series with a variable resistor R₁ across the capacitor 203 whichcontrols a disciminator 208 having the circuit configuration of FIG. 2.The discriminator 208, of course, operates the on-off switch 207 inseries with the machining-current power supply 206. The gap G betweenthe electrodes 201 and 202 also controls the charging rate of capacitor203 in spite of the fact that it is charged by a second DC source 210.In this circuit as well, a swtich may be connected across the capacitor203 to turn on when switch 207 turns off and thereby drain the capacitorupon its discharge state upon the opening of power switch 207 andthereby eliminate charging time variation due to such residual charge. Aswitch of this type has been illustrated in the system of FIG. 5 inwhich the capacitor 303 has the control function as described for thelow-capacitance capacitors of FIGS. 1, 1B and 4. The electrode 301 isconnected to the machining power supply 306 while the on-off switch 307in series therewith is supplied by the discriminator 308 (see thecircuit of FIG. 2) in the manner previously described. The capacitor 303may be charged in series with a switch 311 and the resistor 301 anddischarged when switch 312 is closed through resistor R₂. The switches311 and 312 are operated by the discriminator 308 to turn off and on,respectively, when the switch 307 is turned on by the discriminatornetwork 308. Consequently, when the discriminator circuit 308 turns onswitch 307 a voltage develops across the machining gap from the source306 and initiation of the discharge enables the capacitor 303 to beginto charge to the resistor R₁ with its terminal voltage increasing at arate determined by the resistance of resistor R₁ and the capacitance Cof the capacitor 3 and the gap condition. If the gap has a lowerimpedance with a larger machining discharge current, the capacitorcharges at a faster rate whereas with a smaller discharge current adecreased charging rate results.

The capacitor voltage across capacitor 303 is monitored by the Schmitttrigger circuit and, upon obtaining a preset value set by a referenceresistor of this circuit, the Schmitt trigger provides a switchingsignal to turn switch 7 off and terminate th preceding machiningdischarge. The switch 311 is thereby turned off and the switch 312turned on to enable discharge through the resistor R₂. The capacitor 303discharges and, when its terminal voltage drops to a lower level, thediscriminator 308 threshold is attained to invert the discriminator andcause the switch to turn on and enable the voltage from source 306 to beapplied across the gap and trigger the subsequent discharge.

In FIG. 6 I have shown another embodiment of the invention wherein thecapacitor 403 is charged through the variable resistor R₁ from a source414 other than the source 406 supplying the discharge energy. A switch413 is series with a variable resistor R₂ is connected across thecapacitor 403 and the latter lies in series with the diode 403a incircuit with the gap formed between the tool electrode 401 and theworkpiece 402. The switch 413 is coupled with the power switch 407, inseries with the machining current source 406 and a diode 406a, foroperation by the Schmitt discriminator circuit 408 connected across thecapacitor 403. The switch 413 operates oppositely to the switch 407.

In this circuit, after the switch 407 is rendered conductive and theswitch 413 is turned off, no charging occurs until after a machiningdischarge is initiated at the gap. During this period, the nonconductivestate of the gap open-circuits the capacitor charging network. When thedischarge is initiated at the gap, the latter serving as a switch, thecapacitor 403 is charged from source 414 at a rate determined by theresistance of resistor R₁ and the gap impedance. While the capacitor 403is charged, discharge current passes through the machining gap from thepower source 406 and, when the capacitor voltage reaches a predeterminedmagnitude set at the discriminator 408, the latter inverts (as describedin connection with FIG. 2) to turn off the switch 7 and terminate thedischarge pulse. Simultaneously switch 413 is turned on to discharge thecapacitor 403 through the resistor R₂.

Consequently, if there is a delay in the gap recovery, there will be acorresponding delay in discharge of the capacitor since the voltage fromsource 414 will continue to be applied to the capacitor 403. With a morerapid gap recovery, the source 414 is cut off from the capacitor 403 atan earlier instant to reduce the discharge time since then only thestored charge need be dissipated. When the capacitor 403 terminalvoltage drops to a lower predetermined level, the discriminator circuit408 again responds to turn switch 407 on and switch 413 off to initiatethe next pulse discharge. As in the embodiment of FIG. 5, the circuit ofFIG. 6 uses a capacitor connected in series with the machining gap andthus charging and discharging characteristics control the switching ofthe machining energy. The machining pulse interval, pulse width andrepetition rate are thus accommodated to the varying gap conditions and,since the power pulse is switched, a generally rectangular waveform isproduced.

In FIG. 7 there is shown a modification of the system of FIG. 6 whereina timer 515, constituted by a monostable multivibrator is interposedbetween the discriminator 508 and the switch 507. Here the electrode 501and the workpiece 502 define the machining gap G which receives amachining discharge from a machining current source 506 via the switch507 and a diode 605a while the auxiliary source 514 can charge thecapacitor 503 through the variable resistor R₁. THe discharge resistoris represented at R₂ in series with the switch 513 ganged with theswitch 507 as described in connection with FIG. 6, a diode 503a beinginterposed between rectifier 503 and the tool electrode 501.

In this system, the discriminator 508 responds to the attainment of apredetermined level of the terminal voltage of capacitor 503 to actuatethe timer 515 to turn and hold the switch 507 off and the switch 513 onfor a given period. The capacitor is charged during the machiningoperation and turns the switch 507 off to terminate the gap dischargepulse. At this instant, the timer is energized and upon lapse of apredetermined time thereafter returns the switch 507 to its conductivecondition. While the switch 507 is off, switch 513 is on to permit thecapacitor 503 to discharge through its resistor R₂. Only the chargingportion of the capacitor cycle is thus provided to control the switch.

In the modification of FIG. 8, in which the elements 601 through 603,606-608, 613 and 614 have the same structure and function as thecorresponding elements 501 through 503, 506-508, 513 and 514, comprisesa clock pulse power supply 616 and an AND gate 617 at the output of thediscriminator 608 to operate the switch 607 to pulse the latter. Thedischarge energy is thereby controlled in the form of a succession oftime-spaced trains of brief pulses. The discriminator 8 may directlyenergize a unit-pulse generator, e.g. an oscillator, in which case thegate is omitted.

In all of the embodiments of FIGS. 1 through 8, the capacitor isconnected in series or parallel with the machining gap and its chargingand discharging process provides the switching signals for an on-offswitch. The capacitor is therefore used to control the timing of thepulses and the discharge energy derives from a source other than thecapacitor.

In FIG. 9 I show a circuit using integral control as will be apparent.The circuit basically comprises a tool electrode 701 and a workpiece 702defining the machining-pulse power supply 703 in series with a diode703a and an electronic switch 704 here shown as an NPN transistor forconvenience. As in the previous embodiments, when the switch 704 istriggered into an on state, the discharge current passes substantiallyinstantaneously and is cut off substantially instantaneously when thedischarge is terminated. Any of the switch systems previously describedmay be used for the electronic switch 704.

A separate voltage supply 705 in series with a diode 705a is connectedacross the gap to trigger the discharge or to supply sufficient currentto detect a predischarge phenomenon even where source 703 is cut offfrom the gap in the off condition of switch 704. The circuit of DCsource 705 includes an integrating circuit 706 consisting of a resistor706a in parallel with a capacitor 706b, the integrator lying acrosss theemitter/collector terminals of a transistor 708. A Schmitt-triggerdiscriminator 707 (see FIG. 2) produces an output when the capacitorterminal voltage reaches the predetermined reference or inversionvoltage of the Schmitt circuit. The signal turns on the transistorswitch 708 whose emitter/collector terminals are connected across theintegrating capacitor and thereby discharge the latter.

A second discriminator circuit 709, likewise a Schmitt trigger as shownin FIG. 2, detects the energization of the gap discharge by respondingto the increased voltage level tapped across a resistor 709a in serieswith the source 705. A gating circuit 710 is operated by thediscriminators 707 and 709 to provide a gating pulse to the switch 704and trigger the latter. The gating circuit 710 may be flip-flop orbistable multivibrator as described in PULSE, DIGITAL AND SWITCHINGWAVEFORMS discussed earlier. The integrator 706 and discriminators 707and 709 of the gating circuit 710 thus form the on-off control circuitfor the switch 704. The control signal for the transistor 708 is appliedvia a diode 708a.

Before the discharge is initiated, the switch 704 is in an off conditionso that the output voltage from the power source 703 is ineffectiveacross the machining gap. The high voltage output from source 705 is,however, always effective across the gap (although this source isincapable of making a significant contribution to the machiningcurrent). When the high voltage initiates a discharge, the discriminator709 responds to the small circuit current indicative of the pilotdischarge and produces an output to the gating circuit 710 signaling theexistance of a gap condition capable of effecting machining. The switch704 is turned on rendering the machining voltage from source 703effective and the machining discharge is thus created as soon asbreakdown of the gap by source 705 has been initiated.

The current passing through power circuit 705, 705a, 701, 702, 706 and709a is integrated by the integrator network 706 to develop thereacrossan integrated value represented by the terminal voltage of the capacitor706b. When the voltage reaches a predetermined level established by theSchmitt trigger 707 connected across the integrator 706, the output ofthe discriminator circuit 707 applies a signal to the flip-flop 710 toturn switch 704 off and terminate the machining discharge. An outputpulse from the discriminator 707 is also applied to the switch 708 todrain residual charge from the capacitor and permit the process to berepeated. The pulse discharges have a variable pulse width which dependsinversely on the gap current and hence have substantially equal energyin spite of varying gap conditions. The energy value may, of course, beadjusted, by setting the current magnitude and by varying thecharacteristics of capacitor 706b in the manner previously described.

Instead of current signal detection of the discharge iniation, a changein the gap voltage may be monitored. In this case, the discriminator 709would be connected across the gap to respond to the voltage drop as thedischarge is initiated.

In FIG. 10 there is shown a modification of the system of FIG. 9 in thesense that the integrator circuit 806, consisting of theparallel-connected resistor 806a and capacitor 806b is provided inseries with the electronic switch 804 and the machining power source 803and its diode 803a across the workpiece 802 and the tool electrode 801defining the gap G.

As in the embodiment of FIG. 9, however, the discharge-initiating source805 (pilot-voltage source) is connected across the electrodes 801, 802in series with the rectifier 805a and the sensing resistor 809a of theSchmitt trigger discriminator 809. The output of the discriminator 807,which responds to the integral signal appearing across the capacitor806b, is applied to the gating circuit 810 which may be a flip-flop aspreviously indicated. Another output from the Schmitt-trigger circuit807 is applied via the diode 808a to the base of the capacitor-shortingtransistor 808.

The system of FIG. 10, therefore, differs from that of FIG. 9 in that inthe former the integrated signal was a current representing the gapcondition and derived from a source other than the machining currentsource. In the instant embodiment, the discharge is triggered inresponse to a current from an auxiliary source but is turned off inresponse to the direct integral of the machining current. Otherwise thesystem of FIG. 10 operates in the same manner as that of FIG. 9.

FIGS. 11 through 13 illustrate another concept embodying the basicprinciples set forth earlier and characterized by digital control.

More specifically, the circuit of FIGS. 11 and 12 comprises an electrodeand workpiece system, represented generally at 901 and defining adischarge-machining gap to which a current pulse is applied by theon/off operation of a switch means represented diagrammatically as atransistor 902 whose emitter-collector terminals are in series with themachining power source 903 and a diode 903a as previously described. Inthis system there is also provided a device for monitoring the conditionof the gap or a so-called gap sensor including an auxiliary voltagesource 904 in series with a voltage divider 905, a portion of which isapplied across a capacitor 905a. The result is a terminal voltage at 906which is applied as an input to converter 905, a portion of which isapplied as an input to a converter 907 detailed structurally in FIG. 12.The analog signal, of course, is representative of gap current and themagnitude of the latter is a function of the gap characteristics.

The converter 907 is designed to transform the analog signal into atrain of digital pulses of, for example, a pulse width of 1 to 2microseconds. Consequently, it is an analog/digital converter (A/Dconverter) and may be of conventional design.

The particularly advantageous A/D converter 907 (FIG. 12) comprises anamplifier 971 to intensify the analog signal at terminal 906, and anintegrating network 972 consisting of a capacitor 972a and resistors972b and 972c. The integrating capacitor has its output applied to abistable device 973 such as a Schmitt trigger (FIG. 12) whose statedepends upon the terminal voltage of the capacitor 972 in the mannerpreviously described. The output of the bistable device 973 is designedto invert through a NAND gate 973 providing an output for actuating amonostable multivibrator 975 (PULSE, DIGITAL AND SWITCHING WAVEFORMS) toproduce signal pulses at the OUT terminal as shown. The output of theinverter 974 is also applied to a NOT gate 976 and then through thelatter to a NAND gate 977 receiving an output from the monostablemultivibrator 975. The output of the NAND gate 977 is inverted by anamplifying transistor 978 adapted to control switching transistor 979connected across the capacitor 972 to short-circuit the latter when theswitch is turned on.

The A/D converter 907 feeds into a counter circuit 908 (see pages 668and 683 of PULSE, DIGITAL AND SWITCHING WAVEFORMS) adapted to count thedigital pulses produced by the converter 907 and having an outputselector switch 909 whose function it is to select among the counteroutputs.

A gate circuit 910 is connected through an amplifier 911 with the powerswitch 902 to control the switching operation of the latter. Amultivibrator 912 is provided to set the off-time of the switch 902 at alevel sufficient for gap dielectric recovery while an oscillator 913provides clock pulses of a given frequency, e.g. 10 MHz, to a counter914, the output of which is applied to a logic circuit 915 coupling theoutput of the gate circuit 910 with the output of the counter 914. Afurther coupling (logic) circuit 916 ties the output of circuit 915 tothe output of the circuit 908 to provide a signal representative of ashort-circuited gap, an arcing condition or another undesired conditionof the machining gap. This signal is hereinafter referred to as "U"(unsatisfactory). A delay circuit 917, e.g. a monostable, multivibratoras described at pages 415 through 438 of PULSE, DIGITAL AND SWITCHINGWAVEFORMS, delays for a period the output from the off-time timer 912 tothe gate 910 to stretch the off-time of switch 902 on detection of a Usignal. NAND gates and AND gates according to the present circuit aredescribed at pages 330-334 and 317-321 of PULSE, DIGITAL AND SWITCHINGWAVEFORMS respectively.

The sensing voltage 904 and the sense resistor 905 are continuouslyconnected across the machining gap to monitor the gap current whichvaries in accordance with the gap resistance and provide an analogsignal related to the gap current at the terminal 906. The terminalvoltage at 906 is applied to the A/D converter 907 to charge thecapacitor system at 973 such that when the threshold thereof is reached,the Schmitt circuit inverts to provide a 1 digital signal to the NANDgate 974; the NAND gate 974 has an 0 output which is inverted by the NOTgate 975 to 1 to cause the transistor 978 to be conductive through the 0output of NAND circuit 977 and render transistor 979 conductive toshort-circuit the capacitor 972.

The output of the NAND gate 974 is also applied to the monostablemultivibrator 975 to begin a timing interval of 1 to 2 microsecondsthereby providing to the NAND gate 977 an 0 signal to turn the NANDoutput to 1. Transistors 978 and 979 are turned off to permit thecapacitor 972 to recharge with the input from terminal 906.

When the charging voltage on the capacitor 972 reaches the referencelevel established by the Schmitt circuit 973, inversion again occurs andthe capacitor 972 is discharged.

The repetition of these operations results in the formation of digitalpulses of a pulse width of 1 to 2 microseconds at the OUT terminal ofthe multivibrator 975. The frequency of these pulses is proportional tothe analog signal detected at terminal 906. As the magnitude of theanalog signal increases, the capacitor charges at an increased rate,which, in turn increases the frequency of the digital pulses of theoutput of monostable multivibrator 975.

The digital pulses thus generated are fed to the counter 908 which isreset when it counts up to the number of pulses established by thesetting of the selector network 909 to thereby set the gate circuit 910.When pulses arrive at an increased frequency the counter 908 reaches itssetting before the gate circuit 910 is set and, conversely, when thepulses arrive at a decreased frequency, the counter 908 takes longer toaccumulate the preset counter and the gate circuit 901 arrives at itsset condition before the count is complete.

While the gate circuit 910 is in its reset stage, the NOT gate or theinverter 910a, the NAND gate 910b and the NAND gate 910c have 0, 1, and0 outputs respectively. When the signal from counter 908 indicates theselected count, these gates reverse their outputs to 1, 0, 1respectively. Accordingly, the interval between the time at which thegate 910 is reset and the time at which it is set represents a gatepulse signal which is inverted at amplifier 911 and holds the powerswitch 902 in its conductive condition.

When the gate circuit 910 switches its output to 1 it triggers themonostable multivibrator 912 into operation and turns off the powerswitch 902 for the duration of operation of the multivibrator 912. Theoff-time is set at multivibrator 912 for a period sufficient to allowdielectric recovery of the gap from a preceding discharge.

Upon the lapse of this off-time, the NAND gate 910c in the gate circuit910 receives a reset 1 signal from multivibrator 912 through circuit 917to change its output to 0 and again turn on the power switch 902.

However, when the lapse of the off-time established by multivibrator 912is not sufficient because of a sustained or arc discharge orshort-circuiting at the gap, the delay network of circuit 917 iseffective to stretch the off-time and prevent excessive application ofthe discharge current.

To this end, when the gate circuit 910 produces the output 1 the counter914 is enabled to provide an 0 output which is inverted by a NOT gate orinverter 915a to provide an output 0 at the NAND gate 915b. The outputof NAND gate 915c is thus 1. If there is a short-circuit or otherexcessively low impedance condition in the machining gap 901, anincreased gap current will be detected and a higher-level analog signalwill appear at the terminal 906. The pulses of converter 907 willincrease in frequency and the counter 908 will reach the set count in ashorter time. As a result, the counter 908 provides an 0 signal beforethe counter 914 reaches its output value and the NAND gate 916a willapply a 1 output to the NAND gate 916b so that the latter will have an 0output. The 0 output of the circuit 916 represents the "U" signal toindicate that the gap dielectric condition has not fully recovered fromthe preceding discharge. This signal is applied to the delay circuit 917and delays the reset signal of gate circuit 910 for a period designed toincrease the off-time as previously noted. During this off-time, themachining gap 1 may be adjusted as described in connection with FIG. 1A,or FIG. 1B to modify the gap width or replace the dielectric in the gapmore rapidly, or, in general to correct a parameter of the gap andenable a normal machining condition to resume.

When the gate circuit 910 is reset to again turn power switch 902 on themachining gap 901 is ready for the next dielectric breakdown. Theprocess is then repeated. When counter 908 counts the preset number ofpulses the circuit 910 is set to turn the switch off and as a resultsubstantially rectangular waveform machining pulses are obtained.

The determination of the pulse width is here made on a per pulse basisby counting pulses derived from a signal representing gapcharacteristics, checking the duration against a preset time interval tothereby modify the off-time of the system. The pulse width is thus madeoptimal for the gap conditions and stabilized machining is ensured.

Referring to FIG. 13, the overall approach of this circuit can beappreciated more readily. In general, an electric discharge is createdupon the dielectric breakdown of the liquid dielectric flooding themachining gap. Before a full breakdown is reached a minute current flowswhich avalanches into the full breakdown. The preavalanche phenomenon ishere detected by the monitoring circuit 904, 905 although minute and theterminal at 906 reflects the monitoring current which is converted intodigital pulses accumulated by the counter 908. Upon initiation of thedischarge, the gap monitoring voltage V as represented by the signal atterminal 906, has a buildup proportional to the discharge currentbuildup characteristics. In FIG. 13, the curves I, II and III representrespectively, rapid, moderate and slow buildup of the monitoring voltagedepending upon the gap conductivity when the discharge is initiated.These analog signals are converted into digital signals respectively ofhigh frequency, moderate frequency and low frequency. A similarvariation is characteristic of the discharge and the frequency of thedigital output pulses of the converter depends upon the varyingmagnitudes of the signal at terminal 906. The counter 908 counts up in ashorter time to provide a set signal to the gate circuit 910 toterminate the discharge with a narrower width in the case of thecondition represented by the curve I. In the case of curve III, the 908has an increased count time and hence increases the discharge pulsewidth. The digital signals are thus counted or integrated and thedischarge pulse is cut off upon the integrating value reaching a givenlevel. Discharge pulses of uniform energy are obtained and ready controlof the system by the use of the selector switch 909 is provided.

The timer 913, 914 thus represents a checking timer for control of thedelay circuit 917 to ascertain the off-time.

FIG. 14 represents an embodiment of the invention which can beconsidered to constitute another modification of the integral controlsystem previously described. In this circuit, the tool electrode 1002and the workpiece 1003 are juxtaposed to form the gap G across which themachining power source 1001 is connected in series with an electronicon-off switch 1004 and a resistor 1005 which effectively can be tappedto derive a signal representing the discharge current.

A Miller integrator circuit 1009 (page 536 ff of PULSE, DIGITAL SWITCHNGAND WAVEFORMS) is connected to detect the discharge characteristics andis formed by a resistor 1006, a capacitor 1007 and, in parallel with thelatter, an amplifier 1008. The output of the Miller integrator 1009 isapplied to a comparator, the output of the comparator 1010 is applied toa timer 1011 establishing the pulse interval or off-time. A bistabledevice or flip-flop 1012 has a set terminal connected to the timer 1011and a reset terminal R connected to the comparator 1010. Terminals 1013and 1014 provide start and stop signals, respectively, for operation ofthe flip-flop.

A Schmitt-trigger circuit 1015 (FIG. 2) is also connected to the powercircuit and has its output applied to a differentiator network 1016(pages 38 to 42 of PULSE, DIGITAL AND SWITCHING WAVEFORMS) and to oneinput of a gate circuit 1019 which may either be an INHIBIT gate (pages325 ff or PULSE, DIGITAL AND SWITCH WAVEFORMS) or an AND gate as will bedescribed hereinafter. A diode 1017 is connected by the output ofdifferentiator 1016 and a delay network 1018 adapted to establish athreshold discharge time duration to. The output from gate circuit 1019is applied to a counter 1020 to count the output pulses of the gatecircuit while another counter 1021 counts the output pulses of the delaynetwork 18. These counters may have outputs to a system as described inmy applications Ser. No. 272,463, now U.S. Pat. No. 3781507 and Ser. No.338,849, now U.S. Pat. No. 387374, to control parameters of the gap aswill be apparent hereinafter.

The gate circuit 1019 may either be an INHIBIT gate or an AND gate.Where an INHIBIT gate is used, the counter 20 selectively accumulates acount of discharge pulses whose durations are less than the thresholddischarge duration to established by the delay network 1018, namely,unfavorable pulses. Where the gate circuit 1019 is constituted by an ANDgate, the counter 1020 is adapted to selectively count favorable pulsesor machining pulses other than unfavorable pulses.

Before machining operation is initiated, the flip-flop 1012 is in thereset state so that there is no output therefrom. When the flip-flop1012 receives a set signal from the starting command terminal 1013, itis switched to provide an output and trigger the switch 1004, theconduction thereby initiating the machining pulse substantiallyinstantaneously. The discharge current traversing the gap and the switch1004 induces a voltage E across the resistor proportional to thedischarge current and applied as an input to the pulse duration controlcircuit and to the discrimination and counting circuit. The input to theMiller integrator 1009 is converted into a ramp voltage EM = -E/RC twhere R is the resistance of resistor 1006, C is the capacitance ofcapacitor 1007 and t is the time. The ramp voltage is applied to thecomparator 1010 when it reaches the reference level Es, provides atrigger pulse to reset the flip-flop and turn off switch 1004, therebyterminating the discharge. Simultaneously, the comparator output resetsthe integrator and triggers the timer 1001 into operation. The timerestablishes a predetermined pulse interval in which the switch 1004 isheld nonconductive and after this time, the set signal is applied fromtimer 1011 to the flip-flop so that a new discharge can commence. Theenergy content of each pulse may be selected by adjustment of theresistance of resistor 1006, the capacitance of capacitor 1007, theresistance of resistor 1005 and the gap short-circuiting current.

The voltage across the resistor 1005 also forms the input to the Schmitttrigger 1015 in which it is shaped into a rectangular waveform and therectangular wave is applied to the gate circuit 1019 and to thedifferentiator 1016. The output of the differentiator 1016 is rectifiedby the diode 1017 to produce a signal representing the leading edge ofsuch discharge pulse and forming an input to the delay network 1018which produces a checking pulse. The checking pulse arrives at a time toafter receipt of the leading edge signal and a reply to both the gatecircuit 1019 and the second counter 1021. When an INHIBIT gate is used,pulses of a duration less than the threshold value are accumulated incounter 1020 and where an AND gate is used, pulses of satisfactoryduration are accumulated in counter 1020. Counter 1021 of course, countsall pulses and, when the ratio of unfavorable pulses to favorable pulsesbecomes excessive over a predetermined sequence of pulses, parameteradjustment is effected automatically in accordance with the principlesset forth in the copending applications mentioned earlier.

The counter 1021 is cleared to starting when it has counted a givennumber of input pulses, e.g. ten, and counter 20 may likewise clear tostart when it has counted a given number of input pulses less than thatof counter 1020, say three where the gate 1019 passes unfavorablepulses. The counters 1020 and 1021 can be coupled so that clearing ofcounter 1021 resets counter 1020. Thus, when the counter 1020 hasreceived three unfavorable pulses before receiving the clear signal, itprovides an output used to control a gap parameter and restore properEDM machining.

I have discovered that it is possible to utilize the aforedescribedprinciples for control of an electrical discharge machining apparatus byconverting the analog signal representing a current through the gapduring each machining discharge to a number of countable pulses.

For example, in FIG. 15 I have shown a tool electrode 1101 which isspacedly juxtaposed with a workpiece 1102 across the machining gap G,the electrodes 1101, 1102 being connected in series with a directcurrent machining power source 1103 and a power switch 1104 hereschematically shown as a transistor and described previously.

The discharge circuit also includes, in series with the gap, a resistor1105 adapted to develop a voltage drop proportional to the gap-dischargecurrent, the voltage being applied via the capacitor 1106 to an analogdigital converter 1107 which may have the configuration illustrated inFIG. 17.

The analog digital converter 1107 generates pulses of a frequency whichis a function of the input signal magnitude.

A frequency divider 1108 is connected to the output of the analogdigital converter 1107 and provides an output adapted to be rejected ina presettable counter 1109 whose reset terminal is shown at R. Theoutput from the counter 1109, when the latter reaches its preset count,is applied to a timer network 1110 defining the off-time of the systemand with a flip-flop 1111 whose output energizes and controls (triggers)the power switch 1104 between a conductive and a nonconductivecondition. The reset and set terminals of the flip-flop 1111 arerepresented respectively at R and S and the state of the flip-flop maybe selected by applying a reset signal at terminal 1111a or a set signalto the terminal 1111b.

When the "start" command signal is received at the terminal 1111b, theflip-flop 1111 is set to provide an output 1 to turn the switch 1104into a conductive state while resetting the counter 1109 by the input toits terminal R. A machining voltage from source 1103 is built up acrossthe machining gap G to initiate a discharger therethrough. As soon asthe discharge is initiated, resistor 1105 develops a voltage drop Ewhich is proportional to the discharge current I. High-frequencycomponents contained in the discharge are eliminated by the shuntcapacitor 1106 and only the direct-current component is passed to theanalog/digital converter 1107. The output of the latter has a frequencyf which is applied to the divider 1108 bringing the pulse rate down to asuitable countable rate.

The pulses from frequency divider 1108 are applied to counter 1109which, upon registering the preset value, produces a short-durationsignal pulse which is applied to the reset terminal R of flip-flop 1111and thereby turns the switch 1104 off to terminate the discharge. Thesignal is also applied to the timer 1110, which may be a delay line sothat the output pulse of the latter is applied after the lapse of theselected off-time, to the set terminal S of the flip-flop 1111 andthereby commences another discharge cycle by rendering the switch 1104conductive. The output pulse from the timer 1110 is also applies to thereset terminal of counter 1109 to preset the latter.

The voltage E across the resistor 1105 is represented by

    E =  I/k.sub. 1                                            (1)

whereas the output frequency f of the analog/digital converter 1107 isproportional to the voltage so that

    f = E/k.sub.2                                              (2)

where k₁ and k₂ are adjustable constants obtained by varying theresistance of resistor 1105 or adjusting the analog/digital converter1107.

The dividing ratio of divider 1108 can be represented as 1/n and thepreset number of counter 1109 is represented at N. The number of pulsesN is a summation of the pulses received from the divider or ##EQU2##over a period t, equation (3) being equivalent to ##EQU3## If equations(1) and (2) are substituted in equation (4), one obtains ##EQU4##

Solving for the current integral one obtains ##EQU5## Since N, n, k₁ andk₂ are all constants which may be adjusted with ease, the system mayrespond to any desired current integral and is particularly suited foraccurate response to meet a wide range of machining requirements.

In FIG. 16 I have shown a circuit in which a high-voltage, low-currentsource 1213 is connected across the machining gap in series with a highohmic resistor 1214 (current-limiting resistor) and a sensing resistor1215. The discharge-initiating network also includes a diode 1213apermitting current flow only in the direction represented by the arrowI₂. The electrode 1201 and the workpiece 1202 define the machining gap Gand also lie in series with the low-voltage high-current source 1203 andthe electronic switch 1203 which is rendered conductive once breakdownhas been effected in the gap by the voltage of source 1213. A diode 1212is likewise included in the machining current circuit to confine thecurrent flow to the direction represented by the arrow I₁. The diode1212 blocks the high voltage of source 1213.

As in the embodiment of FIG. 15, the output of resistor 1215 is fed toan analog/digital converter 1207 whose output is applied to a frequencydivider 1208 which feeds the counter 1209. In this embodiment, however,the off-time determination is made by a clock-pulse generator 1216, e.g.an oscillator, whose output is applied to a frequency divider 1217which, in turn, feeds a counter 1218.

The high-voltage auxiliary source 1213 is connected continuously withthe gap and has an output voltage which may be several times that of themachining power source 1203 which may have a potential of, say, 100volts. During the time in which the switch 1204 is nonconductive, thesource 1213 is capable of effecting the discharge but any discharge willdisappear instantaneously because of the drop provided by the high omicresistor 1214 and does not cause any detriment to the machining processas long as there is no arcing at the gap. One or more insignificantdischarges may take place during the off-time of switch 1204 withoutimpeding the actual machining operation.

When switch 1204 is rendered conductive, gap breakdown by source 1214 isfollowed by a substantially instantaneous current flow forming themachining discharge from source 1203. Upon the turning off of powerswitch 1204 the machining discharge terminates and current flow throughthe gap from auxiliary source 1213 again becomes insignificant.

The advantage of the system of FIG. 16 over that of FIG. 15 is thatpower losses across the resistor 1105 of FIG. 15 are avoided.Furthermore, in order to permit the machining current to be at a highlevel, the resistor 1105 must have a relatively low value and thisintroduces the possibility of error. Furthermore, the actual currentflow lags behind the discharge initiation slightly and the circuit ofFIG. 16 responds immediately upon discharge initiation rather thanawaiting the passage of the machining current.

The clock pulse generator of FIG. 16 provides a continuous output ofclock pulses of a fixed and known frequency, the frequency divider 1217transforming the clock pulses to a countable level. The counting pulsesare applied to the counter 1218 which is of the preset type and islocked during the time in which the preset counter 1209 is effective,i.e. during passage of the machining current through the gap. Thusduring this period the preset counter 1218 is effective to provide a 1at its output terminal 1218a and 0 at its output terminal 1218b.

When preset counter 1209 has reached its preset count, the output 1209adevelops a 1 state while its other output terminal 1209b yields ashort-duration pulse which is applied to the reset terminal R of counter1218 thereby releasing the locking state of the latter and permitting itto count incoming clock pulses. The signal at terminal 1218a is thusswitched to 0. When the counter 1218 has registered the preset number ofclock pulses (after division) it switches to provide a 1 at output 1218aand terminal 1218b triggers a pulse which is applied to the resetterminal of counter 1209 and enables the latter to begin counting forthe duration of the next discharge period.

While the counter 1209 has an 0 signal appearing at its output terminal1209a or is in the counting state, a NAND gate 1220 receives this 0signal at one input terminal and provides a 1 output regardless ofwhether its other terminal receives an 0 or a 1 input. The output signalof NAND gate 1220 is amplified at 1221 to turn on and hold the powerswitch 1204 in its conductive state.

When preset counter 1209 registers its preset value the terminal 1209adevelops a 1 while the terminal 1218a is switched to 0, the output ofNAND gate 1219 is switched to 1 and triggers the NAND gate 1220 into its0 output state, thereby turning switch 1204 off to terminate thedischarge.

The switch 1204 is held off until counter 1218 has accumulated thepreset number of incoming clock pulses. The off-time is thus determinedand adjusted by the controlling output frequency of the clock pulsegenerator 1216, the divider ratio of divider 1217 and the preset countof counter 1218.

FIG. 17 shows an analog/digital converter 1107 or 1207 for producingpulses of a frequency proportional to the input voltage. In thisembodiment, the resistor 1215 develops a direct current voltage whosemagnitude is proportional to the varying discharge current traversingthe machining gap as noted previously. This voltage is applied to anoperational amplifier 1271 adapted to develop a voltage at its outputwith zero input or zero discharge current through the gap. The output ofthe operational amplifier 1271 is supplied to an oscillator adapted toprovide pulses of a given frequency as long as it receives an input; thefrequency, however, is modified in accordance with the output voltage ofamplifier 1271.

The oscillator comprises NOT gates or inverters 1272a and 1272b, 1272cand diodes 1273a and 1273b form a series circuit with the diodes poledin one direction and alternating with the inverters. A capacitor 1274 isconnected across the input of

NOT gate 1272b and the input of diode 1273b while a pair of transistors1275a and 1275b have their emitters coupled by this capacitor, theirbases conductively connected and their emitter tied by bias resistorsand a forwardly biased diode 1275c to the output of the operationalamplifier 1271.

The output of the oscillator is fed to a first input terminal of a NANDgate 1276 whose output is fed via an inverter or NOT gate 1277 to thedivider 1208 or directly to the counter 1209 previously described. TheNAND gate 1276 has a second input 1278 which is adapted to receive an 0signal to disable the gate when no input signal appears at sensingresistor 1215, i.e. when no discharge current traverses the gap andthere is null leakage current.

When a discharge current signal is sensed at resistor 1215, theoperational amplifier 1271 has an input voltage which continuouslyvaries in accordance with the voltage drop at resistor 1215 and hence inaccordance with the magnitude of the discharge current. The outputfrequency of the oscillator is thus a function of the current. Thesecond input 1278 of NAND gate 1276 enables the circuit to pass thevariable frequency output as soon as a signal develops in the machininggap. Contact 1279a and 1279b represent relay contacts which may beprovided to switch the oscillator between a variable frequency and aconstant frequency mode. Thus if contacts 1279a are opened and contacts1279 are closed, the system will act as a constant frequency oscillator.

In FIG. 18, I have shown a circuit which is generally similar to FIG. 16but has an additional gap monitor as will be apparent. As in theembodiment of FIG. 16, this circuit comprises a tool electrode 1201, aworkpiece 1202 spaced therefrom across the machining gap G, a powerswitch 1204 in series with the gap and a machining power supply 1203 anda diode 1212 in this power circuit.

In the discharge initiating circuit there is, as previously described, ahigh-voltage low-current source 1213, a current limiting resistor 1214,the diode 1213a and the sensing resistor 1215. In addition, the clockpulse generator 1216, the divider 1217, the delay-setting counter 1218,the NAND gates 1219 and 1220, the amplifier 1221, the analog/digitalconverter 1207 (FIG. 17) the divider 1208 and the counter 1209 all areconnected as has been described in connection with FIGS. 16 and 17.

The gap monitoring circuit provided here in addition is designed suchthat, when the gap voltage falls below a predetermined threshold value,say 12 volts, the high voltage source is shunted from the machining gap.

The shunt circuit comprises a transistor 1228 of the NPN type whoseemitter-collector terminals bridge the network consisting of source 1213resistor 1214 and diode 1213a. The base of transistor 1228 is connectedbetween resistor 1237 and 1238 of a voltage divider tied between thepositive terminal of a direct current source and the collector of a PNPtransistor 1227. The base signal is applied to this transistor from thecollector of another NPN transistor 1226, via a resistor 1234, theemitter of transistor 1226 being connected to the emitter of transistor1225 whose collector is tied to the base of transistor 1226 via the RCnetwork consisting of a capacitor 1224 and a resistor 1231. Areverse-biased diode 1229 leads from the base of transistor 1225 to theelectrode terminal of the system. The unit also, as shown, shunts themain source 1203 simultaneously.

As long as normal machining proceeds in the gap G, there issubstantially no voltage drop and hence the aforedescribed thresholdlevel is not passed, the high voltage source being continuouslyconnected during discharge or the discharge interval. However when ashort-circuit occurs in the gap, the voltage across the latter will fallbelow the threshold and the same holds true when continuous arcingconditions develop.

Under normal operating conditions, the transistor 1225 is maintainedconductive by a voltage source 1222 and transistors 1226, 1227 and 1228are consequently non-conductive. The resistance of resistor 1229 in thebias network of transistor 1225 is adjusted such that, when the gapvoltage drops below the threshold of 12 volts, the transistor 1225becomes non-conductive and transistors 1226, 1227 and 1228 are renderedconductive. The source 1213 is thereby shunted. Upon recovery of the gapvoltage to a level above 12 volts, transistor 1225 is returned to theconductive state. Bias resistors are provided in accordance with theusual practice at 1230 through 1237.

I claim:
 1. A method of controlling the machining of a workpiece by theapplication of machining pulses across a dielectric-swept gap betweensaid workpiece and an electrode spacedly juxtaposed therewith, themethod comprising the steps of:switching a machining current powersupply on and off in circuit with said gap to apply machining dischargesthereacross; integrating a signal representing the gap current passingthrough said gap after initiation of a discharge thereacross to producea control signal; and effecting the switching of said power supply uponthe magnitude of said signal attaining predetermined value, said controlsignal being produced by converting a signal representing the gapcurrent into a train of digital pulses and integrating said train ofdigital pulses.
 2. The method defined in claim 1 wherein said pulses ofsaid train are accumulated in a counter to produce said control signal.3. A method of controlling the machining of a workpiece by theapplication of machining pulses across a dielectric-swept gap betweensaid workpiece and an electrode spacedly juxtaposed therewith, themethod comprising the steps of:switching a machining current powersupply on and off in circuit with said gap to apply machining dischargesthereacross; integrating a signal representing the gap current passingthrough said gap after initiation of a discharge thereacross to producea control signal; effecting the switching of said power supply upon themagnitude of said signal attaining predetermined value; timing theduration of each machining discharge; counting the number ofunsatisfactory machining discharges having a duration less than apredetermined duration over a predetermined number of sequentialmachining discharges; and modifying a parameter of said gap upon thecount of unsatisfactory machining discharges exceeding a predeterminednumber.
 4. A method of controlling the machining of a workpiece by theapplication of machining pulses across a dielectric-swept gap betweensaid workpiece and an electrode spacedly juxtaposed therewith, themethod comprising the steps of:switching a machining current powersupply on and off in circuit with said gap to apply machining dischargesthereacross; integrating a signal representing the gap current passingthrough said gap after initiation of a discharge thereacross to producea control signal; effecting the switching of said power supply upon themagnitude of said signal attaining predetermined value; timing theduration of each machining discharge; counting the number ofsatisfactory machining discharges having a duration within apredetermined duration range and thereby ascertaining the number ofunsatisfactory discharges having a duration outside said range; andmodifying a parameter of said gap upon ascertaining unsatisfactorydischarges exceeding a predetermined amount.
 5. In a system for theelectric discharge machining of a workpiece by an electrode juxtaposedtherewith across a dielectric-filled gap, the improvement whichcomprises in combination:an electric-current source; on-off switch meansconnected in circuit with said source, said workpiece and said electrodeand triggerable to apply substantially rectangular-wavemachining-current pulses to said gap to effect electrical dischargethereacross; an integrator adapted to generate a control signalrepresenting the integral of a current flow through said gap; meansresponsive to said integrator for operating said switch means upon saidsignal attaining a predetermined value; means for detecting the durationof the successive discharges formed by the repetitive operation of saidswitch means; means for determining unsatisfactory discharges of aduration outside a predetermined duration range; and means for producinga signal for controlling a parameter of the gap upon the number ofunsatisfactory discharges exceeding a predetermined number over asequence of predetermined number of discharges.
 6. The improvementdefined in claim 5 wherein said means for producing a signal forcontrolling said parameter includes a counter for accumulating a countof the number of satisfactory discharges for forming the latter signal.7. The improvement defined in claim 5 wherein said means for producing asignal for controlling said parameter includes a counter foraccumulating a count of the number of unsatisfactory discharges.
 8. Theimprovement defined in claim 5 wherein said means for determiningdischarges of a duration outside said range includes a timer energizedat the leading flank of each discharge, gating means responsive to saidtimer, and a counter connected in circuit with said gating means foraccumulating a count of discharges.